Thermistors, which are ceramic resistors sintered from mixtures of metal oxides, typically have 20 times the temperature sensitivity, one tenth the cost, and twenty times the resistance of purely metallic resistance-sensing elements. High thermistor resistance values allow simple two wire connections to a measurement instrument without concern about variations in lead resistance.
Over a typical thermistor range of -80 to +150 degrees Celsius, a thermistor can have excellent conformance to published curves and further exhibit repeatability that approaches that of metallic resistance elements. However, as is known, a thermistor's curve of resistance-versus-temperature is much more nonlinear than that of a metallic resistance element. Therefore, thermistors require a meter with linearizing capabilities.
The equation that closely matches a thermistor/temperature response curve is the Steinhart-Hart equation: EQU 1/T=A+B*1n(R)+C*[1n(R)].sup.3 ( 1)
where A, B, and C are selected to match a particular thermistor type. At higher temperatures, the Steinhart-Hart equation also includes a [1n(R)].sup.2 term. The "B" term in equation 1 is by far the dominant variant term for most thermistors.
To convert thermistor resistance values to accurate thermistor temperature indications over the full useful temperature range, a typical meter must utilize means to compress the typical 40,000:1 thermistor resistance span into a high accuracy/high resolution input span capable of being handled by a meter, typically 10:1. The prior art has employed a number of techniques to accomplish or avoid span compression.
Certain meters have added resistors whose values are chosen to enable sensing of a particular temperature span. Those resistors place the thermistor in a bridge configuration and enable the meter to appropriately sense chosen middle values, while causing the meter to lose sensitivity at temperature extremes. Other meters have employed a pair of matched thermistors to obtain an almost linear differential signal for a restricted temperature range. Still others have switched the thermistor drive current (either up or down manually or by autoranging) until the thermistor signal is within the meter's accurate range. Finally, a meter's input voltage scale can be switched, either manually or by autoranging an input voltage divider, so as to enable the input voltage to be within the meter's accurate range.
The above described techniques cause one or more of the following: limit the measurement range; limit the speed of response of the meter; significantly raise sensor and/or meter costs; or result in high voltages or high currents with concomitant sensor heating effects and reduced accuracies.
It has been further realized in the prior art, that because the resistance of a thermistor varies exponentially with thermistor temperature, that a logarithmic detector will provide compensation for both the exponential characteristic and the wide variation in thermistor resistance values. In this regard, it has been proposed that a diode-like semiconductor be employed as the logarithmic detector, with the diode-like semiconductor provided by a bipolar transistor whose collector is connected to its base electrode to create a "quasi-diode". The collector voltage of a quasi-diode is given, to high accuracy, by the Arrhenius equation (with ohmic drop added) as follows: EQU Vm=(kTm/q)*(1n[I]-1n[Irs])+I*Rq (2)
where:
Tm is the absolute temperature of the quasi-diode junction in kelvins PA1 k is Boltzmann's constant; PA1 q is the electronic charge; PA1 I is the current flowing through the thermistor; PA1 Irs is the reverse-saturation current of the bipolar transistor; and PA1 Rq is the ohmic resistance of the bipolar transistor. PA1 Ci is a constant for a particular transistor, depending upon geometry and doping densities; PA1 x is a power whose value depends on the mobility of electrons in silicon, reported as having a value from 2 to 4; PA1 Vg is the computed gap voltage for the conduction-valence band separation in silicon (usually 1.21 volts).
The logarithmic relationship of equation 2 translates current in the nanoampere-to-milliampere range to a 200-to-700 millivolt range for a typical meter temperature environment. This useful translation however, has not been used in the prior art because of the large effects of meter temperature on the quasi-diode voltage, particularly as a result of the nearly-exponential increase in reverse saturation current of the diode with meter temperature.
A theoretical reverse-saturation current of a transistor, exhibiting a uniform base with negligible base recombination and uniform doping, is given by equation 3 below: EQU Irs=Ci*(Tm.sup.x)*e.sup.(Vg/[kTm/q]) ( 3)
where:
Reverse saturation current is known to increase more than 1000:1 over a typical meter's operating range. Since the voltage appearing at the collector of a quasi-diode is: EQU V=kT/q*1n(I/Irs) (3a)
it can be readily seen that quasi-diode voltage varies substantially with reverse saturation current variations.
Accordingly, it is an object of this invention to provide an improved resistance-sensing meter that employs a logarithmic detector to counteract an exponential characteristic of a sensing resistor.
It is another object of this invention to provide an improved temperature sensing meter that employs a thermistor temperature sensor, the temperature sensing meter including a logarithmic current detector circuit.
It is yet another object of this invention to provide an improved temperature sensing meter that employs a thermistor and logarithmic sensor wherein outputs from the logarithmic sensor are temperature compensated so as to provide accurate temperature readings.